In the configuration of blowing moulds comprising two mutually displaceable half-moulds (it being possible for the mould also to have a third part formed by a stationary or axially movable mould bottom), the two half-moulds are firmly fastened to each other during blowing. Thus, in moulds of the mutually articulated type commonly used in installations sold by the Applicant, the two half-moulds are articulated so as to rotate about a common axis parallel to one side (located to the rear) of the mold parting surface, whereas in the closed position of the mould clamping means mechanically fasten the two half-moulds together along the opposite side (located to the front) of the mold parting surface.
However, despite the very great mechanical rigidity obtained in the closed position by the moulds thus designed, it often happens, owing to the high blowing pressures commonly employed (in practice around 40×105 Pa), that the two half-moulds move apart (gaping) in the upper part of the mold parting surface, on the upper face of the mould that has the opening for passage of the neck of the container treated (in the lower part of the mould, the presence of the mould bottom, fastened to the two half-moulds by tongue-and-groove assembly means, prevents this drawback).
Admittedly, this drawback is put to good use since it is in particular by this gaping that the air filling the moulding cavity can escape, this air being forced out by the increase in volume of the container during blowing. In addition, it is common practice for the edges of the upper contacting faces of the half-moulds, which constitute the upper part of the mold parting surface, to be specifically machined so as to facilitate venting via this means.
Nevertheless, the fact remains that this gaping in the upper part of the mould constitutes a drawback that practitioners have always desired to avoid, the venting air from the moulding cavity having to be able to be evacuated via other means (for example machined venting channels through the wall of the half-moulds).